Various systems have been developed for dispensing measured quantities of liquids by many different means. The most precise types use a positive displacement pump, usually with a controlled stroke piston, to fill and empty a container of the required volume with each stroke. The piston can be driven by mechanical means, such as a crank drive or similar reciprocating device, or by compressed air or the like. With mechanical drives the apparatus is bulky, complex and requires considerable maintenance. In apparatus operated by high pressure means, it is difficult to control leakage and avoid contamination of the liquid by propellant gas. Some systems dispense several liquids selectively through a common outlet, in order to reduce the bulk of the apparatus and permit the use of common drive means. Contamination is again a problem and breakdown of one component may require shut down and servicing of the entire system.